One prior art flash memory device is a stack gate flash EEPROM where a single stack-gate transistor constitutes the memory cell. It programs as a traditional UV-erasable EPROM, using the mechanism of hot-electron injection to a floating gate, and erases through Fowler-Nordheim tunneling mechanism from the source region. Such device suffers the disadvantages of (1) over-erase sensitivity, where the memory cell can be erased to a negative threshold voltage thus rendering the cell in a conductive state even when the gate of the cell is deselected and biased at a ground potential, and (2) high programming current, which requires the memory cell to be programmed by a separate power supply voltage. See for example, U.S. Pat. No. 4,698,787.
A second type of flash memory device utilizes a split gate configuration. This eliminates the over-erase sensitivity, because even if the floating gate is over-erased, conduction in the channel requires the biasing of the control gate which is over another portion of the channel. However, the programming and erase mechanisms are the same as the stack-gate configuration. The disadvantage of this configuration is that it increases the cell size and suffers an alignment sensitivity because of the split gate arrangement. See for example, U.S. Pat. No. 5,029,130.
Yet another type of flash memory cell utilizes the so called source-side injection technique which minimizes the hot electron programming current to the extent that an on-chip voltage multiplier can be used to provide sufficient programming current from a single 5 or 3.3 V power supply. However, the structure of these cells still suffers from (1) alignment sensitivity, (2) poor scalability and (3) compromise between cell size and coupling ratio. See U.S. Pat. No. 5,194,925.
Finally, reference is made to U.S. Pat. No. 4,462,090 and 5,280,446 which disclose a single transistor memory cell having four terminals with a select gate, a control gate, and a source and drain. The memory cell disclosed in those references, however, uses a split gate configuration for the select gate. Such a split gate configuration for the select gate can cause punch through sensitivity due to misalignment.